There are a number of circumstances in which it is necessary or desirable to accomplish a relay operation with signals that are applied to the relay by a signal transmission line that is used in the transmission of other signals. That gives rise to the problems of how to prevent the presence of a relay in the circuit from having adverse affect on those other signals, how to distinguish the relay control signal and how to control and accomplish both switching and resetting. One example is found in the requirement to use electric power or telephone lines in the selective, remote, shut down of energy consuming apparatus. Another example of the problem occurs in telephone systems.
The telephone line that is used to communicate voice signals must also be used to transmit tones and to operate coin return switches and a telephone ringer. Several elements or pieces of apparatus are connected to the line. Each is operated with a different kind of signal--all transmitted through the same line. It is required to keep each piece of apparatus from degrading signals intended for other pieces of apparatus, and to prevent each piece of apparatus from responding except to its own signal. That problem has existed in telephone systems for a long time. Signals, and the apparatus that responds to them, have been developed to entirely compatible form. That was possible because of standardization within the telephone industry itself, and because of protective laws that prohibited intrusion by others into the telephone lines.
That situation is changed by recent developments in the law. Telephone company customers now have the privilege of connecting their own telephone systems and their own modum and computer and patching interface equipment to the telephone lines. When a customer experiences difficulty with his telephone system, he may not be able to determine whether or not it is the telephone line itself or the apparatus that he has connected to the system which is the cause of the trouble. It is far easier to lodge a complaint with the telephone company than to attempt to find the manufacturer of the modum or the non-standard telephone or other non-standard apparatus to which the telephone line is connected. The costs to telephone companies of servicing such complaints can become excessive. A means is required at the interface between the telephone line and the customer supplied equipment by which the telephone company can determine with relative ease and minimum expense whether, its line or the customer supplied equipment is defective.
Some means is required for conducting a test of the line without testing or effecting the customer supplied apparatus. That can be done by disconnecting the telephone line from the customer supplied apparatus and connecting it, instead, to a test apparatus. That test apparatus can be passive or active. A passive apparatus would consist of a load of known electrical impedance. An active apparatus would be an alternating current generator whose frequency is changed across the pass band of the telephone line during a test.
Either of those schemes requires disconnection of the customer's apparatus from the telephone line for the period of the test. The problem can be simplified if the test apparatus, instead of being connected to the line continuously, is connected only during the period of the test. Of course, it is preferable that the test be conducted at the customer's facility where the line joins the customer supplied apparatus. Any useful solution of the problem will permit the test to be conducted remotely from a telephone company's central station or test facility.
Switching at the line end from customer apparatus to a test apparatus can be accomplished with a relay at the line end if a way can be found to operate the relay remotely. The telephone line itself can be used to send one or more signals that operate the relay to disconnect the customer supplied apparatus, connect the test apparatus to the line, and, when the test is complete, disconnect the test apparatus and reconnect the customer supplied apparatus. That arrangement will remove the test apparatus from the line except during the test period. However, the signal sensing element of the relay that is responsive to the relay operating signal must remain connected to the line. One of the requirements of the signal sensing element is that it not load the telephone line. The electromagnetically operated relay that is conventional in the telephone system can not meet that requirement. Its impedance is simply not high enough and it uses too much power. Another arrangement must be found.
There are several kinds of apparatus that will accomplish switching and that exhibit high electrical impedance. There is an electrostatic relay that exhibits high impedance. It utilizes electrical actuation and mechanical switch operation. Examples of such switches can be seen in U.S. Pat. No. 4,093,883 to Yujiro Yamamoto. Another, and more obvious, choice is solid state electronic switching. Neither choice is without problems. Any test will require some time period to complete. Separate "start-test" and "return-from-test" signals may be used, or a timer at the line end may be used to provide automatic return-from-test after a time delay. Neglecting cost and size considerations, incorporating the timer is not difficult in either the solid state or electrostatic relay. In the case of electrostatic relays that employ a deformable piezo-electric element to produce mechanical contact movement, delay is introduced by limiting discharge rate. But that produces slow mechanical actuation of the relay contacts and, in a telephone system where voltages are relatively high, the result is arcing that shortens contact life. The electrostatic approach presents other problems as well. Nonetheless, the solution which this invention provides has the electrostatic relay as its base.